The physical characteristics of cellulosic materials such as fabrics and paper can often be enhanced by chemical treatment which causes crosslinking of cellulose molecules. In the case of fabrics, such treatments can provide wrinkle resistance, shrinkage resistance, smooth drying and crease retention properties and in the case of paper, improved wet strength.
Treatments with formaldehyde-based reagents, such as dimethyloldihydroxylethyleneurea (DMDHEU), have been used in the past to impart these valuable characteristics on cellulosic fabrics because the formaldehyde-based reagents are relatively inexpensive and effective. Urea-formaldehyde and melamine-formaldehyde reagents are used as wet strength agents to enhance the wet performance of paper. However, formaldehyde is an irritant and a known carcinogen. In addition, cellulosic fabrics treated with formaldehyde-based reagents suffer severe strength loss.
Thus, finding non-formaldehyde crosslinking agents for cellulosic fabrics has been the focus of the textile finishing industry since the mid-1980's. Polycarboxylic acids are among the new crosslinking agents studied. A number of polycarboxylic acids have been effective as durable press finishing agents.
For instance, 1,2,3,4-butanetetracarboxylic acid (BTCA) provides effective crosslinking for cellulosic textiles thereby imparting high levels of wrinkle resistance and laundering durability. While there have been extensive efforts to use BTCA as a non-formaldehyde durable press finishing agent to replace traditional formaldehyde-based reagents, the prohibitively high cost of BTCA has prevented its commercial scale application in the textile industry.
While it is known that polycarboxylic acids having their carboxyl groups bonded to adjacent carbons of the molecular backbone are more effective for esterifying cellulose than polycarboxylic acids with carboxyl groups bonded to alternative carbons; such polycarboxylic acids are not as effective as needed for commercial purposes.
Citric acid has also been tested as a crosslinking agent for cellulosic fabrics. However, the fact that citric acid is only moderately effective coupled with poor laundering durability and fabric yellowing makes the use of citric acid undesirable.
A cost effective non-formaldehyde crosslinker which provides both satisfactory crosslinking and good fabric strength retention is greatly needed. The present invention satisfies this need. The Applicant has discovered a unique combination of relatively low priced polymeric carboxylic acids and .alpha.-hydroxypolycarboxylic acids which, when reacted in situ in the presence of a catalyst, provides superior cellulosic crosslinking as well as strength retention for cellulosic fabrics. Treatment of paper with the combination of polymeric carboxylic acids and .alpha.-hydroxy polycarboxylic acids also significantly improves the wet performance of the treated paper.